This invention relates generally to a beam forming network for circular polarization, and, more particularly, to a beam forming network for a circularly polarized beam antenna system for communication satellite applications.
A shaped beam antenna system for communication satellite applications typically comprises an offset paraboloid reflector, waveguide horn feed array and beam forming network (BFN) to eastablish the desired feed array illumination characteristics for antenna beam shaping. The BFN is a microwave network for coupling elements that interfaces the individual elements of the feed array with the communication satellite transponder.
The coupling elements utilized in the BFN are usually of the terminated, quadrature type in order to minimize the sensitivity of the antenna system to internally generated reflections, and allow the antenna designer the freedom to select nonuniform distributions for the shaped beam optimization. The desired feed array phase distribution produced by the BFN is typically required to be constant, or, in other words, the phase of the signals from individual elements at the output of the antenna array should be of the same phase. To correct for the quadrature phase differences introduced by the coupling elements fixed phase shifters must be introduced in the network.
A typical BFN with quadrature couplers is shown in FIG. 1 where the required phase compensation is introduced at the coupler level. Note the number of phase shifters between couplers and at the output of the BFN. The use of such phase shifters between the couplers to correct for the phase takes up a great deal of space and adds significantly more weight. Both weight and space are at a premium in satellite applications. It is therefore desirable to find some way in which the overall size and the weight of the BFN can be reduced.